This invention relates generally to detecting misfire in internal combustion engines, and more particularly to detecting misfires attributable to a cause in the fuel supply system.
In spark-ignition internal combustion engines, the spark plug in conjunction with other ignition system components generates a discharge arc across the spark gap from one electrode to another. The challenge is to discharge the firing spark at just the precise moment and under the right conditions to ensure ignition of the air-fuel mixture introduced into the combustion chamber. When the air-fuel mixture does not ignite, for whatever reason, a "misfire" has occurred.
Misfires are generally attributable to a failure in either the ignition system or fuel supply system. In misfires attributable to the ignition system, the problem is usually a failure to generate the required firing spark which may be caused, for example, by a fouled spark plug. Misfires attributable to fuel system failures typically are associated with a problem in supplying the proper air-fuel mixture to the combustion chamber. Regardless of the cause, the effects of misfiring are typically the same: increased fuel consumption, degraded engine performance, and inefficiencies in the catalytic converter which may result in increased emissions. Accordingly, it is desirable to detect misfires in order to make adjustments to minimize their occurrence.
Several prior art approaches are known for detecting misfires attributable to a cause in the fuel supply system. U.S. Pat. No. 5,215,067 teaches a system for detecting misfires attributable to a cause in the fuel system. In one embodiment, the system monitors the primary coil discharge voltage during a predetermined time period. The system then calculates a value proportional to the area in which the sensed voltage exceeds a reference value. This value is then compared with a threshold value. In one embodiment when the former exceeds the latter, a misfire is immediately determined to have occurred. In another embodiment, the system repeatedly carries out the above procedure until the predetermined time period elapses at which point the comparison to the threshold value is made to determine the occurrence of a misfire.
The inventors herein have recognized several problems and disadvantages with the approach taught in U.S. Pat. No. 5,215,067 and other time based methods and systems. For example, the duration and magnitude of the firing spark is highly dependent upon engine operating conditions such as temperature and engine load. Accordingly, in order to ensure accurate detection of misfire across a broad spectrum of engine operating conditions, significant empirical study must be performed to determine the appropriate time periods and threshold values for comparison purposes. Moreover, more memory may be required to store these reference values in a look-up table for future use.
Another disadvantage associated with the approach taught in U.S. Pat. No. 5,215,067 is that in order to derive a value proportional to the area in which the sensed voltage exceeds a reference value during the initial capacitive discharge state may require a very fast sampling rate. Accordingly, there may be a trade-off between sampling rate and system accuracy.